Multi-pollutant exhaust treatment using seawater for marine applications

ABSTRACT

Marine engine exhaust includes pollutants such as CO 2 , NO x  and SO x . An on-board system and method for the simultaneous removal of these pollutants includes obtaining seawater from the water on which the marine vessel travels, purifying the seawater to remove a portion of hard ions, concentrating the seawater to yield a concentrated brine solution, treating the concentrated brine solution with a chemical softener to yield a treated brine solution, acidifying the treated brine solution, and utilizing the acidified brine solution in a chlor-alkali process to yield sodium hydroxide. The sodium hydroxide can be used in an acid gas scrubber to remove CO 2 , NO x , and SO x  from the marine engine exhaust gas.

TECHNICAL FIELD

This disclosure relates to treating pollutants in the engine exhaust ofa marine vessel using seawater.

BACKGROUND

Emissions derived from maritime transport and inland shipping includecarbon dioxide (CO₂), nitrogen oxides (NO_(x)), and sulphur oxides(SO_(x)). Environmental studies have shown that marine diesel enginesare the largest atmospheric polluters of the two latter: NO_(x) andSO_(x).

Recent international regulations and commitments seek to limit exhaustgas emissions, however, there is little experience in shipping relatedemission-reducing technologies, relative to the industrial and energysector. Moreover, in the shipping industry, the removal of NOx, SO_(x)and CO₂ from exhaust gases is commonly done separately. Combining thesemethods, such as combining SO₂ scrubbing together with selectivecatalytic reduction (SCR) followed by CO₂ capture, is extremely costly.Therefore, the effective, efficient and simultaneous removal ofpollutants from engine exhaust gas is an important part of marinecommerce.

SUMMARY

This disclosure describes a method and apparatus for removing multiplepollutants from the engine exhaust gas of a marine engine usingseawater.

Certain aspects of the subject matter described here can be implementedas a method for treating engine exhaust gas emitted by a marine engineonboard a marine vessel. On board the marine vessel, a portion ofseawater on which the marine vessel travels is obtained. The portion ofthe seawater is treated to remove portions of calcium, magnesium andsulfate ions from the portion of the seawater to yield a treated portionof seawater. The treated portion of the seawater is concentrated toyield a concentrated brine solution. The concentrated brine solution istreated with a softener to yield a treated brine solution. The treatedbrine solution is acidified to yield an acidic brine solution. Theacidic brine solution is processed with a chlor-alkali unit to yieldhydrogen gas, chlorine gas and sodium hydroxide. The sodium hydroxide isreacted in a scrubber unit with the engine exhaust gas to remove carbondioxide, SO_(x), and NO_(x) from the engine exhaust gas.

An aspect, combinable with any of the other aspects, can include thefollowing features. The treated portion of the seawater is concentratedto yield potable water.

An aspect, combinable with any of the other aspects, can include thefollowing features. The treated portion of the seawater is concentratedwith one or more of a reverse osmosis process, an electrodialysisprocess, a multi-stage flash distillation process, a multi-effectdistillation process, or a mechanical vapor compression process.

An aspect, combinable with any of the other aspects, can include thefollowing features. The treated portion of the seawater is concentratedwith an electrodialysis process powered by a fuel cell. The fuel cell ispowered with hydrogen gas produced by the chlor-alkali unit.

An aspect, combinable with any of the other aspects, can include thefollowing features. The treated portion of the seawater is concentratedwith a mechanical vapor compression process. The mechanical vaporcompression process includes a compressor powered by the engine thatcompresses vapor produced by evaporation. The mechanical vaporcompression process includes a preheating heat exchanger that recoversheat from the compressed vapor. The mechanical vapor compression processincludes an evaporator that is powered by the recovered heat andevaporates the treated portion of the seawater to yield the concentratedbrine solution.

An aspect, combinable with any of the other aspects, can include thefollowing features. The concentration of sodium chloride in the treatedportion of the seawater is increased to up to 26% by weight to yield theconcentrated brine solution.

An aspect, combinable with any of the other aspects, can include thefollowing features. Chlorine and hydrogen gas generated by thechlor-alkali process are converted into hydrochloric acid. Thehydrochloric acid is used to acidify the treated brine solution.

An aspect, combinable with any of the other aspects, can include thefollowing features. The chlorine gas is reacted with sodium hydroxide toyield sodium hypochlorite.

An aspect, combinable with any of the other aspects, can include thefollowing features. The hydrogen gas generated by the chlor-alkaliprocess is combusted in the marine engine.

An aspect, combinable with any of the other aspects, can include thefollowing features. The sodium hydroxide is reacted with the engineexhaust gas in the scrubber unit to yield sodium bicarbonate.

An aspect, combinable with any of the other aspects, can include thefollowing features. The sodium bicarbonate is used as the softener totreat the concentrated brine solution.

An aspect, combinable with any of the other aspects, can include thefollowing features. The engine exhaust gas passes through a waste heatrecovery unit before the engine exhaust gas is reacted with the sodiumhydroxide. Heat is recovered from the engine exhaust gas by the wasteheat recovery unit.

An aspect, combinable with any of the other aspects, can include thefollowing features. The heat recovered from the waste heat recovery unitis utilized by the mechanical vapor compression process.

An aspect, combinable with any of the other aspects, can include thefollowing features. The sodium hydroxide and the engine exhaust gasreact to yeild solid and liquid products and byproducts. These solidsand liquids are separated using a separation unit.

An aspect, combinable with any of the other aspects, can include thefollowing features. The sodium hydroxide is reacted in a scrubber unit.The scrubber unit is at least one of a packed tower, wet-film,impingement-plate or tray-tower scrubber.

An aspect, combinable with any of the other aspects, can include thefollowing features. A system for treating an engine exhaust gas emittedby a marine engine onboard a marine vessel uses a portion of seawaterfrom the seawater on which the marine vessel travels. A treatment unitremoves portions of calcium, magnesium and sulfate ions from the portionof the seawater to yield a treated portion of the seawater. One or moreconcentration units concentrate the treated portion of the seawater toyield a concentrated brine solution. A chemical softening unit furtherremoves calcium and magnesium ions from the concentrated brine solutionto yield a treated brine solution. An acidifying unit acidifies thetreated brine solution to yield an acidic brine solution. A chlor-alkaliunit electrolyzes sodium chloride in the acidic brine solution to yieldhydrogen gas, chlorine gas, and sodium hydroxide. An acid gas scrubberreacts the sodium hydroxide with the engine exhaust gas. The solid andliquid products and byproducts of the reaction between sodium hydroxideand the engine exhaust gas are separated by a separator.

An aspect, combinable with any of the other aspects, can include thefollowing features. The one or more concentration units include at leastone of a reverse osmosis, an electrodialysis, a multi-stage flashdistillation, a multi-effect distillation, or a mechanical vaporcompression unit.

An aspect, combinable with any of the other aspects, can include thefollowing features. The electrodialysis unit is powered by a fuel cell.

An aspect, combinable with any of the other aspects, can include thefollowing features. The fuel cell is powered by hydrogen gas produced bythe chlor-alkali unit.

An aspect, combinable with any of the other aspects, can include thefollowing features. The mechanical vapor compression unit includes acompressor powered by the marine engine, an evaporator, and a preheatingheat exchanger.

An aspect, combinable with any of the other aspects, can include thefollowing features. The chlori-alkali unit produces chlorine gas andhydrogen gas.

An aspect, combinable with any of the other aspects, can include thefollowing features. The acid gas scrubber is at least one of a packedtower, wet-film, impingement-plate or tray-tower scrubber.

The details of one or more implementations of the disclosure are setforth in the accompanying drawings and the description that follows.Other features, objects, and advantages of the disclosure will beapparent from the description and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 shows an example of a marine vessel with an exhaust treatmentsystem.

FIG. 2 shows an example of a marine engine exhaust treatment system.

FIG. 3 shows an example schematic of a chlor-alkali unit.

FIG. 4 is flowchart showing an example method of treating marine enginegas using seawater.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

Provided in this disclosure, in part, are methods and an apparatus fortreating marine engine exhaust using seawater. FIG. 1 shows an exampleof a marine vessel 100 which includes an engine 102 that produces engineexhaust 104. The engine exhaust 104 can be treated by the on-boardtreatment system 106. FIG. 2 shows an example of the on-board treatmentsystem 106. The apparatus for treating the marine engine exhaust can beused aboard a marine vessel 100 that includes an engine 102 whichproduces engine exhaust 104. The apparatus further includes apurification unit 202 and a concentration unit 204. The apparatusincludes a softening unit 206. The apparatus includes a chlor-alkaliunit 208 which can be powered by a fuel cell 209. The chlor-alkaliprocess produces sodium hydroxide (NaOH) 224 which can be used in anacid gas scrubber 210 to remove SO_(x), NO_(x) and CO₂ from the engineexhaust 104. The apparatus can further include a waste heat recoveryunit 260 which recovers heat from the engine exhaust 104 and cools theengine exhaust 104 before it reaches the acid gas scrubber 210. In someimplementations, the apparatus includes a separator unit 212 to separatethe products and byproducts of the acid gas scrubber 210. The apparatuscan also include any tubes, pipes, hosing, connectors, valves, fluidconnections and manual or computer implemented controls necessary tooperate the apparatus.

In some implementations, the on-board treatment system 106 utilizes aportion of seawater 216 obtained from the seawater on which a marinevessel 100 travels. The seawater 216, which naturally contains sodiumchloride, is treated with a purification unit 202. The purification unit202 is configured to remove primarily sulfate ions (SO₄ ²⁻) and aportion of hard ions, for example, calcium (Ca²⁺) and magnesium (Mg²⁺)ions. The removal of calcium, magnesium and sulfate ions improves theefficiency of the subsequent chlor-alkali process. The purification unit202 can purify the seawater 216 using nanofiltration. The purificationunit 202 yields treated seawater 218.

The treated seawater 218 is processed by a concentration unit 204. Theconcentration unit 204 concentrates the sodium chloride in the treatedseawater 218 to yield a concentrated brine solution 220. Theconcentrated brine solution can approach sodium chloride saturation, forexample, approximately 26% by weight sodium chloride. The concentrationunit 204 can use any suitable process to increase the concentration ofsodium chloride. For example, the concentration unit 204 can include aone or more of a reverse osmosis process, an electrodialysis process, amulti-stage flash distillation process, a multi-effect distillationprocess, or a mechanical vapor compression process.

In some implementations, the treated seawater 218 undergoes more thanone concentration process to yield the concentrated brine solution 220.These concentration processes can be multiple iterations of the sameprocess, or combinations of more than one type of process depending onseveral factors, such as the energy requirements, process efficiency,seawater's salinity, equipment sizing, flexibility and others. Forexample, reverse osmosis process can concentrate the treated seawater218 with almost minimum thermodynamic energy consumption, due to theimplementation of more energy-efficient pumps and improved membranes,whereas electrodialysis could be an alternative to reverse osmosis forlow-salinity desalination applications. Moreover, because of the extremeflexibility of the electrodialysis process, it can be coupled with afuel cell which is powered by hydrogen produced by the chlor-alkaliunit. In some implementations, the mechanical vapor compression processconcentrates the treated sweater by compressing vapor produced byevaporation using a compressor powered by the marine engine 102,recovering heat from the compressed vapor using a preheating heatexchanger, powering an evaporator using the recovered heat, andevaporating the treated seawater using the evaporator to yield theconcentrated brine solution.

In some implementations, concentrating the treated seawater 218 yieldspotable water 219. The potable water 219 can be utilized on-board orsold as a commodity.

In some implementations, the concentrated brine solution 220 isprocessed with a chemical softening unit 206 using one or more softenersto yield a treated brine solution 221. The chemical softening unit 206further removes hard ions (for example, Ca²⁺ and Mg²⁺). In someimplementations, the chemical softening unit 206 uses sodium carbonate(Na₂CO₃) and sodium hydroxide (NaOH) to precipitate the Ca²⁺ and Mg²⁺ions as calcium and magnesium salts. In some implementations, a cationion-exchange step can further remove hard ions.

In some implementations, the treated brine solution 221 is acidified,yielding an acidified brine solution 222. The acidified brine solution222 is treated with a chlor-alkali unit 208. An example schematic of achlor-alkali unit is shown in FIG. 3. The chlor-alkali unit 208 includesan electrochemical cell 302. The electrochemical cell 302 includes atleast one anode 304, at least one cathode 306, and an ion exchangemembrane 308. The acidified saturated brine solution 222 enters theelectrochemical cell 302. In the electrochemical cell 302, electrolysisresults in the generation of chlorine gas (Cl₂) 242 at the anode 304,hydrogen gas (H₂) 240 at the cathode 306, as shown in the followingreactions:Anode: 2Cl⁻Cl₂(g)+2e ⁻Cathode: 2H₂O+2e ⁻→H₂(g)+2OH⁻

The ion exchange membrane 308 allows only the diffusion of sodiumcations (Na⁺) from the anode side to the cathode side, where they reactwith the hydroxyl anions (OH⁻) to produce sodium hydroxide 224.

The overall reaction of this process is:2NaCl+2H₂O→2NaOH+H₂+Cl₂

In some implementations, the sodium hydroxide 224 is a solution ofsodium hydroxide.

In some implementations, the sodium hydroxide 224 is used in thechemical softening unit 206.

In some implementations, the chlorine gas 242 and hydrogen gas 240 canbe converted by conversion unit 211. Hydrogen chloride (HCl) gas can beformed in the conversation unit 211 by combustion of hydrogen (H₂) andchlorine (Cl₂) gas. The hydrogen chloride gas is then absorbed by waterto form hydrochloric acid 244. The reaction is represented by theequation:H₂+Cl₂→2HCl

Both the reaction and absorption process in water are highly exothermic.In some implementations, the hydrochloric acid 244 can be used toacidify the treated brine solution 221 to yield the acidified brinesolution 222.

Alternatively or in combination, the chlorine gas 242 can be combinedwith the sodium hydroxide 224 to form sodium hypochlorite (NaOCl), alsoknown as bleach. The bleach can be used on-board the marine vessel 100or sold as a commodity.

Alternatively or in combination, the hydrogen gas 240 can be used topower a fuel cell 209. The fuel call 209 can be used to power part ofthe apparatus. For example, the fuel cell 209 can be used to power anelectrodialysis process in the concentration unit 204.

Alternatively or in combination, the hydrogen gas 240 can be used topower combustion in the marine engine 102.

Alternatively or in combination, the hydrogen gas 240, the chlorine gas242, or both gasses can be sold as a commodity.

In some implementations, the sodium hydroxide 224 produced by thechlor-alkali unit 208 reacts with the exhaust gas 104 from the marineengine 102 in an acid gas scrubber 210. The acid scrubber 210 can be apacked tower unit, wet-film, impingement-plate, tray-tower, or othertype of scrubber unit.

In some implementations, a waste heat recovery unit 260 is used to coolthe engine exhaust 104 to a cooled engine exhaust 105 before the engineexhaust enters the acid gas scrubber 210.

In the acid gas scrubber 210 the sodium hydroxide 224 reacts with CO₂,SOX, and NO_(x) in the engine exhaust gas 104 or alternatively with thecooled engine exhaust gas 105 to yield product and byproducts. Forexample, the SO_(x) and NO_(x) in the engine exhaust gas 104 or cooledengine exhaust gas 105 react with the sodium hydroxide 224 to formsodium sulfates and sodium nitrates, for example, Na₂SO₄ and NaNO₃. TheCO₂ in either the exhaust gas 104 or the cooled exhaust gas 105 reactswith the sodium hydroxide 224 to form sodium carbonate (Na₂CO₃) andsodium bicarbonate (NaHCO₃) 250. Some of the products and byproducts ofthe acid scrubbing process have commercial applications, and can be soldas commodities. For example, the generated sodium bicarbonate 250 can besold as baking soda, used in algae biofuels production, or used in glassmanufacture. Alternatively, the sodium bicarbonate 250 can be safelylandfilled, used for mine backfilling, or used as a road base. Thegenerated sodium sulfate (Na₂SO₄) can be used as filler in laundrydetergents, or used in the glass industry. The generated NaNO₃ can beused for making potassium nitrate, fertilizers, explosives, in theproduction of high-strength glass, in some pharmaceuticals, or as theraw material for the production of nitric acid.

In some implementations, the sodium carbonate 250 generated by the acidgas scrubber unit 210 is used as a softener in the chemical softeningunit 206.

The products and byproducts of the acid gas scrubber unit 210 alsoinclude a mixture of solid and liquid products and byproducts 228 andclean exhaust 226.

In some implementations, the output of the acid gas scrubber unit 210,that is, the mixture of solid and liquid products and byproducts 228,are separated using a separating unit 212. The separating unit 212 canseparate solid products 230 from other products and byproducts, forexample from the spent brine solution 232.

FIG. 4 is a flowchart showing an example method of treating marineengine exhaust 104 using an on-board treatment system 106. At 402,seawater 216 is obtained from the seawater on which the marine vessel100 travels. At 404, the seawater 216 is treated to remove portions ofcalcium, magnesium, and sulfate ions. At 406, the treated seawater 218is concentrated to yield a concentrated brine solution 220. At 408, theconcentrated brine solution is acidified. At 410, the acidified brinesolution 222 is processed with a chlor-alkali unit 208 to yield hydrogengas 240, chlorine gas 242, and sodium hydroxide 224. At 412, the sodiumhydroxide 224 is reacted with the engine exhaust gas 104 to removecarbon dioxide, SO_(x) and NO_(x) from the engine exhaust gas 104.

Reference has been made in detail to certain embodiments of thedisclosed subject matter, examples of which are illustrated in part inthe accompanying drawings. While the disclosed subject matter isdescribed in conjunction with the enumerated claims, it will beunderstood that the exemplified subject matter is not intended to limitthe claims to the disclosed subject matter.

The term “about” as used in this disclosure can allow for a degree ofvariability in a value or range, for example, within 10%, within 5%, orwithin 1% of a stated value or of a stated limit of a range.

As used in this disclosure, “weight percent” (wt %) can be considered amass fraction or a mass ratio of a substance to the total mixture orcomposition. Weight percent can be a weight-to-weight ratio ormass-to-mass ratio, unless indicated otherwise.

A number of implementations of the disclosure have been described.Nevertheless, it will be understood that various modifications can bemade without departing from the spirit and scope of the disclosure.

What is claimed is:
 1. A method for treating engine exhaust gas emitted by a marine engine onboard a marine vessel, the method comprising: onboard the marine vessel: obtaining a portion of seawater on which the marine vessel travels; treating the portion of seawater to remove portions of calcium, magnesium and sulfate ions from the seawater to yield a treated portion of the seawater; concentrating the treated portion of the seawater to yield a concentrated brine solution; treating the concentrated brine solution with a softener to yield a treated brine solution; acidifying the treated brine solution to yield an acidic brine solution; processing the acidic brine solution with a chlor-alkali unit to yield hydrogen gas, chlorine gas, and sodium hydroxide; and reacting the sodium hydroxide in a scrubber unit with the engine exhaust gas to remove carbon dioxide, SO_(x) and NO_(x) from the engine exhaust gas.
 2. The method of claim 1, wherein concentrating the treated portion of the seawater yields potable water.
 3. The method of claim 1, wherein concentrating the treated portion of the seawater comprises concentrating with one or more of a reverse osmosis process, an electrodialysis process, a multi-stage flash distillation process, a multi-effect distillation process, or a mechanical vapor compression process.
 4. The method of claim 3, wherein concentrating the treated portion of the seawater comprises concentrating with the electrodialysis process powered by a fuel cell, wherein the fuel cell is powered with hydrogen gas produced by the chlor-alkali unit.
 5. The method of claim 3, wherein concentrating the treated portion of the seawater comprises concentrating the treated portion of the seawater with a mechanical vapor compression process, wherein the mechanical vapor compression process comprises: compressing vapor produced by evaporation using a compressor, wherein the compressor is powered by the marine engine; recovering heat from the compressed vapor using a preheating heat exchanger; powering an evaporator using the recovered heat; and evaporating the treated portion of the seawater using the evaporator to yield the concentrated brine solution.
 6. The method of claim 1, wherein concentrating the treated portion of the seawater comprises increasing the concentration of sodium chloride in the treated portion of the seawater up to 26% by weight to yield the concentrated brine solution.
 7. The method of claim 1, further comprising converting the chlorine gas and hydrogen gas generated by the chlor-alkali process into hydrochloric acid, and acidifying the treated brine solution using the hydrochloric acid.
 8. The method of claim 1, further comprising reacting the chlorine gas with sodium hydroxide to yield sodium hypochlorite.
 9. The method of claim 1, wherein the hydrogen gas generated by the chlor-alkali process is combusted in the marine engine.
 10. The method of claim 1, wherein reacting the sodium hydroxide in the scrubber unit with the engine exhaust gas yields sodium bicarbonate.
 11. The method of claim 10, wherein the sodium bicarbonate is used as the softener to treat the concentrated brine solution.
 12. The method of claim 1, wherein the engine exhaust gas passes through a waste heat recovery unit before the engine exhaust gas is reacted with the sodium hydroxide, and heat is recovered from the engine exhaust gas by the waste heat recovery unit.
 13. The method of claim 12, wherein the heat recovered from the waste heat recovery unit is utilized by the mechanical vapor compression process.
 14. The method of claim 1, wherein reacting the sodium hydroxide and the engine exhaust gas yields solid and liquid products and byproducts, and further comprising separating the solid and liquid products using a separation unit.
 15. The method of claim 1, wherein reacting the sodium hydroxide in a scrubber unit comprises reacting the sodium hydroxide in at least one of a packed tower, wet-film, impingement-plate or tray-tower scrubber.
 16. A system for treating an engine exhaust gas emitted by a marine engine onboard a marine vessel using a portion of seawater from the seawater on which the marine vessel travels, comprising: a treatment unit configured to remove portions of calcium, magnesium and sulfate ions from the portion of the seawater to yield a treated portion of the seawater; one or more concentration units configured to concentrate the treated portion of the seawater to yield a concentrated brine solution; a chemical softening unit configured to further remove calcium and magnesium ions from the concentrated brine solution to yield a treated brine solution; an acidifying unit configured to acidify the treated brine solution to yield an acidic brine solution a chlor-alkali unit configured to electrolyze sodium chloride in the acidic brine solution to yield hydrogen gas, chlorine gas, and sodium hydroxide; an acid gas scrubber configured to react the sodium hydroxide with engine exhaust gas; and a separator configured to separate the solid and liquid products and byproducts that are generated by the reaction of sodium hydroxide and engine exhaust gas.
 17. The system of claim 16, wherein the one or more concentration units comprise at least one of a reverse osmosis, an electrodialysis, a multi-stage flash distillation, a multi-effect distillation, or a mechanical vapor compression unit.
 18. The system of claim 17, wherein the electrodialysis unit is powered by a fuel cell.
 19. The fuel cell of claim 18, wherein the fuel cell is powered by hydrogen gas produced by the chlor-alkali unit.
 20. The system of claim 16, wherein the mechanical vapor compression unit comprises: a compressor powered by the marine engine; an evaporator; and a preheating heat exchanger.
 21. The system of claim 16, wherein the chlor-alkali unit produces chlorine gas and hydrogen gas.
 22. The system of claim 16, wherein the acid gas scrubber is at least one of a packed tower, wet-film, impingement-plate or tray-tower scrubber. 